The present invention relates in general to oscillators and, more particularly, to a current controlled ring oscillator having a linear output frequency over the control range.
Current controlled ring oscillators are commonly used in electronic circuit design, for example in phased lock loops, for providing an adjustable output frequency operating in the range of say 150 megahertz (Mhz). A common design for ring oscillators uses an odd number of serially connected differential amplifier stages with the differential outputs of one stage connected (+/+ and -/-) to the differential inputs of a subsequent stage. The differential outputs of the final stage are crossed-coupled (+/- and -/+) back to the inputs of the first stage. Thus, a three stage ring oscillator would have 60.degree. of phase shift per amplifier stage followed by 180.degree. due to the cross coupling between the output of the final stage back to the input of the first stage for providing 360.degree. of total phase shift to cause the circuit to oscillate.
To adjust the output frequency, the ring oscillator often utilizes a current control feature in the amplifier stages. More current increases the frequency of the amplifier by charging internal capacitive nodes faster and increasing the transition rate of the amplifier stage. The higher the control current, the faster each amplifier stage transitions and the higher the output frequency of the ring oscillator.
Maintaining a linear output frequency over the current control range is important for accuracy and stability in applications such as phase lock loops. Unfortunately, conventional ring oscillators typically exhibit poor linearity in response to the control current. The current controlled gain of each amplifier stage, defined as the rate of change in frequency with respect to control current (df/di), can vary by as much as 60% over the control range. Ideally the rate of change in output frequency should remain constant over the control current range. Any non-linearity in the output frequency of the ring oscillator degrades the phase lock loop system performance.
Hence, a need exists for an improved current controlled ring oscillator with a linear output frequency as a function of control current.